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Coil for variation in metal

Joris De Beer
Prodigy 80 points

Hello,

We want to detect the variation in thickness of a set metal plates with a diameter of 10 mm.

Each plate will be measured separately and we suppose that the thickness will variate from 100 um to 500 um.

If it is possible we want a resolution of 1 um.

The plates would be placed at 1 mm from the sensor coil.

Is this possible to measure with the LDC1000?

Which sensor coil do you propose for this measurement?

With best regards,

Joris.

  • 0
    TI__Genius 13650 points

    Hello Joris,

    there are different approaches that may be suitable for your system:

    - place target parallel to a PCB sensor coil to measure thickness directly. Since your maximum metal thickness is thin, it may work if the target material and the distance between sensor coil and target is constant. You will need to use a very low sensor frequency. You can find more information in question "What are the general guidelines for sensing a target through metal?" on the inductive sensing FAQ.

    - place metal plate perpendicular to the sensor coil. This approach doesn't have the same restrictions on maximum metal thickness. However, the metal plate still needs to be at a fixed distance above the coil to avoid sensing the target distance instead of the target thickness.

    - translate the metal thickness to a lateral movement. If you can do this mechanically and with the required geometries, this is likely to offer the best resolution. You can find more information on the inductive sensing FAQ under 'Can I use an Inductance-to-Digital converter to measure lateral movement (linear position sensor)?'

  • 0 in reply to Ben Kasemsadeh
    Prodigy 80 points

    Thanks for the fast reply.

    My LC tank that i am working with is a ferrite pot core coil of 680uH (Rs= 2.3ohm) and a capacitor of 33nH.

    The resonance frequency is 33.5kHz (5kHz < fres < 5MHz) and the resonance impedance is 8959 ohm (798 ohm < Rp < 3.93M ohm).

    The RP_MIN register is set to 0.798k ohm, and the RP_MAX register is set to 21.547 k ohm(2 x 8959ohm = 17918 ohm).

    But when I start the measurement there is a lot of noise visible in the proximity data window of the GUI (LDC1000 EVM GUI 1.0.4.1)

    What could be the cause of this noise?

    The resonance frequency and the impedance fall between the limits of the LDC1000.

    Kindest Regards,

    Joris.

  • 0 in reply to Joris De Beer
    TI__Genius 13650 points

    Hi Joris,

    please post a screen capture of the noisy GUI window.

  • 0 in reply to Ben Kasemsadeh
    Prodigy 80 points

    Hello Ben,

    Here are the screen captures from the GUI:

    The configuration settings:

    With best regards,

    Joris.

  • 0 in reply to Joris De Beer
    TI__Genius 13650 points

    Hi Joris,

    did you set the filter capacitor correctly (refer to LDC1000 datasheet section 'Choosing Filter Capacitor (CFA and CFB Pins')? A wrong filter capacitor value could cause noise on Rp but not on L.